<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    let str = 'abcd12345678'
    console.log(str.substring(str.length - 8, str.length))
  </script>

</body>

</html>